by running it into moulds of brass, clay, plaster, &c. But the chief articles in plumbery are sheets and pipes of lead; and as these make the basis of the plumber's work, we shall here give the process of making them. In casting sheet-lead, a table or mould is made use of, which consists of large pieces of wood well jointed, and bound with bars of iron at the ends, on the sides of which runs a frame, consisting of a ledge, or border of wood, two or three inches thick, and two or three inches high from the mould, called the sharps: the ordinary width of the mould, within these sharps, is from three to four feet; and its length is sixteen, seventeen, or eighteen feet. This should be something longer than the sheets are intended to be, in order that the end where the metal runs off from the mould may be cut off, because it is commonly thin, or uneven, or ragged at the end.It must stand very even or level in breadth, and something falling from the end in which the metal is poured in, viz. about an inch, or an inch and a half, in the length of sixteen or seventeen inches. At the upper end of the mould stands the pan, which is a concave triangular prism, composed of two planks nailed together at right angles, and two triangular pieces fitted in between them at the ends. The length of this pan is the whole breadth of the mould in which the sheets are cast; it stands with its bottom, which is a sharp edge, on a form at the end of the mould, leaning with one side against it, and on the opposite side is a handle to lift it up by, to pour out the melted lead; and on that side of the pan next the mould are two iron hocks to take hold of the mould and prevent the pan from slipping, while the melted lead is poured out of it into the mould. This pan is lined on the inside with moistened sand, to prevent it from being fired by the hot metal. The mould is also spread over, about two-thirds of an inch thick, with sand sifted and moistened, which is rendered perfectly level by moving over it a piece of wood called a strike, by trampling upon it with the feet, and smoothing it over with a smoothing plane, which is a thick plate of polished brass, about nine inches square, turned up on all the four edges, and with a handle fitted on the upper or concave side. The sand being thus smoothed, it is fit for casting sheets of lead; but if they would cast a cistern. they measure out the bigness of the four sides, and having taken the dimensions of the front, or fore-part,

make mouldings, by pressing long slips of wood, which contain the same mouldings, into the level sand, and form the figures of birds, beasts, &c. by pressing in the same manner leaden figures upon it, and then taking them off, and at the same time smoothing the surface where any of the sand is raised up, by making these impressions upon it.

The rest of the operation is the same in casting either cisterns or plain sheets of lead; but before we proceed to mention the manner in which that is performed, it will be necessary to give a more particular description of the strike. The strike, then, is a piece of board about five inches broad, and something longer than the breadth of the mould on the inside; and at each end is cut a notch about two inches deep, so that when it is used it rides upon the sharps with those notches. Before they begin to cast, the strike is made ready by tacking on two pieces of an old hat on the notches, or by slipping a case of leather over each end, in order to raise the under side about oneeighth of an inch, or something more, above the sand, according as they would have the sheet to be in thickness; then they tallow the under edge of the strike, and lay it across the mould. The lead being melted, it is ladled into the pan, in which, when there is a sufficient quantity for the present purpose, the scum of the metal is swept off with a piece of board to the edge of the pan, letting it settle on the sand, which is by this means prevented from falling into the mould at the pouring out of the metal. When the lead is cool enough, which is known by its beginning to stand with a shell or wall on the sand round the pan, two men take the pan by the handle, or else one of them lifts it up by a bar and chain fixed to a beam in the ceiling, and pour it into the mould, while another man stands ready with the strike, and, as soon as they have done pouring in the metal, puts on the mould, sweeps the lead forward, and draws the overplus into a trough prepar ed to receive it. The sheets being thus cast, nothing remains but to planish the edges, in order to render them smooth and straight; but if it be a cistern, it is bent into four sides, so that the two ends may join the back, where they are soldered together, after which the bottom is soldered up.

The method of casting thin sheets of lead. Instead of sand, they cover the mould with a piece of woollen stuff nailed down at the two ends to keep it tight,

and over this lay a very fine linen cloth. In this process great regard is had to the just degree of heat, so as that the lead may run well, and yet not burn the linen. This they judge of by a piece of paper, for it takes fire in the liquid lead if it is too hot, and if it be not shrunk and scorched a little, it is not hot enough.

The Method of casting Pipes without soldering. To make these pipes they have a kind of little mill, with arms or levers to turn it withal. The moulds are of brass, and consist of two pieces, which open and shut by means of hooks and hinges, their inward calibre, or diameter, being according to the size of the pipe to be made, and their length is usually too feet and a half. In the middle is placed a core, or round piece of brass or iron, somewhat longer than the mould, and of the thickness of the inward diameter of the pipe. This core is passed through two copper-rundles, one at each end of the mould, which they serve to close; and to these is joined a little copper tube, about two inches long, and of the thickness the leaden pipe is intended to be of. By means of these tubes the core is retained in the middle of the cavity of the mould. The core being in the mould, with the rundles at its two ends, and the lead melted in the furnace, they take it up in a ladle, and pour it into the mould by a little aperture at one end, made in the form of a funnel. When the mould is full, they pass a hook into the end of the core, and, turning the mill, draw it out; and then, opening the mould, take out the pipe. If they desire to have the pipe lengthened, they put one end of it in the lower end of the mould, and pass the end of the core into it; then shut the mould again, and apply its rundle and tube as before, the pipe just cast serving for rundle, &c. at the other end. Things being thus replaced, they pour in fresh metal, and repeat the operation till they have got a pipe of the length required. For making pipes of sheet-lead, the plumbers have wooden cylinders, of the length and thickness required, and on these they form their pipes, by wrapping the sheet around them, and soldering up the edges all around them.

PLUME, a set or bunch of ostrich feathers, pulled out of the tail and wings, and made up to serve for ornaments in funerals, &c. Among sportsmen, plume is the general colour or mixture of the feathers of a hawk, which shows her constitution.

honour of Charles Plumier; a genus of the Pentandria Monogynia class and or der. Natural order of Contorta. Apocineæ, Jussieu. Essential character: contorted; follicles two, reflex; seeds inserted into their proper membrane. There are four species.

PLUMMET, PLUMB-RULE, or PLUMBLINE, an instrument used by carpenters, masons, &c. in order to judge whether walls, &c. be upright planes, horizontal, or the like. It is thus called from a piece of lead, "plumbum," fastened to the end of a cord, which usually constitutes this instrument. Sometimes the string descends along a wooden ruler, &c. raised perpendicularly on another; in which case it becomes a level. See LEVEL.

PLUMMING, among miners, is the method of using a mine-dial, in order to know the exact place of the work where to sink down an air-shaft, or to bring an adit to the work, or to know which way the load inclines when any flexure happens in it. It is performed in this manner a skilful person, with an assistant, and with pen, ink, and paper, and a long line, and a sun-dial, after his guess of the place above ground, descends into the adit or work, and there fastens one end of the line to some fixed thing in it, then the incited needle is let to rest, and the exact point where it rests is marked with a pen: he then goes on further in the line, still fastened, and at the next flexure of the adit he makes a mark on the line by a knot or otherwise; and then letting down the dial again, he there likewise notes down that point at which the needle stands in this second position. In this manner he proceeds from turning to turning, marking down the points, and marking the line, till he comes to the intended place; this done, he ascends, and begins to work on the surface of the earth what he did in the adit, bringing the first knot in the line to such a place where the mark of the place of the needle will again answer its pointing, and continues this till he comes to the desired place above ground, which is certain to be perpendicularly over the part of the mine into which the air-shaft is to be sunk.

PLUMULA, in botany, a little feather, the scaly part of the corculum, or em bryo plant within the seed, which ascends and becomes the stem or trunk. It extends itself into the cavity of the lobes, and is terminated by a small branch resembling a feather, from which it derives

PLUNGER, in mechanics, a solid brass cylinder, used as a forcer in forcing pumps.

PLURAL, in grammar, and epithet applied to that number of nouns and verbs which is used when we speak of more than one thing; or that which expresses a plurality of numbers or things. See GRAMMAR.

PLURALITY. In ecclesiastical matters, no person having one benefice, with cure of souls, of 87. a year, in the King's books, shall accept another; but the former benefice shall be void, unless the person has a dispensation from the Archbishop of Canterbury, who has power to grant dispensations to chaplains of noblemen and others, under proper qualifications, to hold two livings, provided they are not more than thirty miles distant from each other; and provided that he reside in each, for a reasonable time, every year; and that the parson keep a sufficient curate in that in which he does not ordinarily reside.

PLUS, in algebra, a character marked thus, used for the sign of addition.

PLUSH, in commerce, &c. a kind of stuff having a sort of velvet knap, or shag, on one side, composed regularly of a woof of a single woollen thread and a double warp, the one wool, of two threads twisted, the other goat's or camel's hair; though there are some plushes entirely of worsted, and others composed wholly of hair. Plush is manufactured, like velvet, on a loom with three treadles; two of these separate and depress the woollen warp, and the third raises the hair warp, upon which the workman throwing the shuttle passes the woof between the woollen and hair warp; and after. wards laying a brass broach, or needle, under that of the hair, he cuts it thereon with a knife destined for that use; conducting the knife on the broach, which is made a little hollow all its length, and thus gives the surface of the plush an appearance of velvet. See VELVET.

PLUVIAMETER. See RAIN gauge. PNEUMATICS, is that branch of natural philosophy which treats of the weight, pressure, and elasticity of the air, with the effects arising from them.

Galileo, whose name is presented as of itself, whenever the inquiry relates to the first researches concerning gravity, had verified that of the air, which was denied almost universally before him, though it had been discovered by some few philosophers of antiquity. This celebrated philosopher having injected air

into a glass vessel, so that it there remained compressed, found that the vessel weighed more than when the contained air was in its natural state. He inquired also, by another experiment, into the heaviness of this fluid compared with that of water; but he found it only in the ratio of 1 to 400, which is much too small, as we shall soon see. The pneumatic machine, or air-pump, was not then known. It is to Otto Guericke, a burgomaster of Magdeburg, that we are indebted for the invention of this elegant machine, which is not, like many others, confined to one part of experimental philosophy, for almost all branches derive aid from it. This machine, which will be presently described, when reduced to its greatest simplicity, is composed of a vertical cylinder of brass, in which a piston is moved; its upper base carries a cock, above which is soldered a circular brass plate situated horizontally. On this plate the receiver is placed, from which we would exhaust the air, which is executed by making the piston descend and ascend alternately. By the use of this instrument, the gravity of the air has been verified, by first weighing a ball or bladder full of air, and then weighing it, after the ball or bladder has been exhausted of the air; a sensible diminution will be perceived in the weight of the ball. Philosophers have attempted likewise to determine, with precision, the specific gravity of the air.

According to the results of Deluc, the ratio between the weight of common air and distilled water, at the temperature of thawing ice, and under a medium pressure of 29.9 English inches of mercury, is that of 1 to 760; and from the experiments of Lavoisier, it follows that a cubic inch of air, taken at ten degrees of Reaumur, weighs 0.46005 grains, and that the weight of a cubic foot of the same fluid is one ounce, three drams, and three grains, but by some very accurate experiments of Mr. Cavendish, it was ascertained that the weight of water, is to that of air as 800 to 1: this was the case when the barometer stood at 294 inches, and the thermometer at 50°. Sir George Shuckburgh found it to be as 836 to 1, when the barometer was 29.37, and the thermometer at 51°. The medium of many experiments by the gentlemen already mentioned, and by Mr. Hauksbee, Dr. Halley, Mr. Cotes, and other philosophers equally zealous in the improvements of natural science, is about 832 to 1, when the barometer is 30°, and the thermome

ter 55°: this ratio must vary in proportion to the changes in the height of the barometer, and it varies alsoth part for every degree of the thermometer above or below temperature: hence the cubic foot of air, of water, and of quicksilver, may be taken as 1 ounce, 1000 ounces, and 13,600 ounces.

The gravity of the air being once known, it should seem that it could not be difficult to infer that the ascent of water, in the body of a pump, must be occasioned by the pressure of that fluid. This, however, was not the case: Galileo had no notion of it.

Some Italian conduit makers being asked if they would construct sucking pumps, whose tubes should be more than 33 feet in height, remarked, with surprise, that the water refused to rise above that linit. They requested of Galileo the explication of this singular fact; and it is affirmed that the philosopher, being taken unawares, replied, that nature did not entertain the horror of a vacuum beyond 33 feet. Torricelli, a disciple of Galileo, having meditated upon this phenomenon, conjectured that water is elevated in pumps by the pressure of the exterior air; and that this pressure has only the degree of force necessary to counterbalance the weight of a column of water of 33 feet. He verified this conjecture by an experiment, for which natural philosophy owes him a double obligation, since it serves to render evident an important discovery, while it has procured us the barometer. Torricelli saw the mercury stand 29 or 30 inches in a glass tube, sealed at its upper part, and situated vertically; and the height thus under consideration being to that of 33 feet in the inverse ratio of the densities of water and of mercury, he concluded that the phenomenon belonged to statics, and that it was really, as he had conjectured, the pressure of the air which caused water or mercury to rise until an equilibrium was produced: this occurred in 1643. The year following, the news of Torricelli's experiment was desseminated in France by a letter written from Italy to Father Mersenne. The experiment was performed again in 1646, by Mersenne and Pascal; and the latter devised, in 1647, a method of rendering it still more decisive, by making it at different altitudes. He invited, in consequence, his friend Perrier to repeat the experiment upon the mountain Puy-de-dome, and to observe whether the column of mercury

would descend in the tube in proportion as it became more elevated. We may see from the letter of Pascal to Perrier, where he seems to avoid the name of Torricelli, that he had not yet entirely renounced the chimera of the horror at a vacuum which was attributed to nature, and that by admitting that this horror was not invincible, he was not bold enough to assert that it never obtained. The success of the experiment completely removed the delusion. Yet this experiment was only a confirmation of that by Torricelli, and therefore yielded an additional ray to the stream of light which issued from it. The pressure of the atmosphere, upon a given surface, being nearly the same as would be exerted upon that surface by a column of water of 33 feet high, from this datum has been computed the effect of the pressure under consideration, with respect to a man of medium magnitude, and it has been found that it is equivaient to a weight of about 33,600 pounds. Considerable as this weight is, its pressure is exerted unknown to us, because it is continually balanced by the reaction of the elastic fluids comprised in the interior cavities of our bodies; and though the air is subject to continual variations, which augment or diminish its density, in consequence of changes of temperature, and of the action of different natural causes, yet, as these variations are generally confined within narrow limits, and succeed each other with comparative tardiness, they do not affect us commonly, except in a manner scarcely perceptible. But if there happen a sudden change, as when a man is raised to great heights, the rupture of the equilibrium which ensues has a very marked influence upon the animal economy. He then experiences an extreme fatigue, and absolute inability to continue his progress; a drowsiness, under which he sinks in spite of himself; the respiration becomes thick and difficult; the pulsations take an accelerated motion. To explain these effects, it must be considered that the state of well-being, in all that depends upon respiration, requires that a determinate quantity of air should pass through the lungs in a given time. If, therefore, the air that we respire becomes much more rare, the inspirations must of necessity be proportionally more frequent; which will render the respiration more difficult, and will occasion the various symptoms to which we have referred. With regard to the inconveniences that

would result from an air too condensed, man is not exposed to them by the action of natural causes; and it appears that, in general, they are less than those which are caused by the rarefaction of the air.

We need only cite here, as a proof of the small magnitude of these inconveniences, that which happens to divers, when they have been shut up within a bell which descended vertically in the water, and in which the air, pressed by the weight of the surrounding columns, contracts itself more and more, in proportion as the vessel is found at a greater depth. The accidents which have occurred to those who have continued for a certain time under the bell have arisen, in great part, from the alteration produced in the air by respiration, and that which was most dangerous in this fluid was the defect of renewing it. See DIVING bell, BAROMETER, &c.

One

The elasticity of the air is verified by several well-known experiments. of the most ordinary is that in which we employ the machine called the artificial fountain. It consists of a metallic vessel of a rounded form, its summit being pierced with an orifice, through which the vessel may be filled with water to about twothirds of its capacity. In this aperture a tube is then fixed, which descends into the vessel until it is within a little distance of the bottom, while its upper part, which projects from the orifice, is furnished with a cock. To this same part a forcing pump is adapted, and the cock being opened, a great quantity of air is injected into the vessel: this air, being lighter than water, rises above it, and its elasticity augments with its density, in proportion as new strokes are given to the piston. Then, after closing the cock, the pump is removed, and a kind of little hollow cone is substituted for it, open at its summit, which is turned upwards; as soon as the cock is again opened, the condensed air exerts its force upon the surface of the water, and drives it through the canal that is immersed into that liquid, whence it is seen to shoot out under the form of a jet of more than twenty or thirty feet in height. An analogous effect may be obtained, solely by diminishing the natural elasticity of the air, by placing under the receiver of an air pump a little vessel, in which all is similar to what the artificial fountain presents at the moment when the cock is opened to give a free passage to the water,

except that the air situated above this liquid is in its ordinary state.

While the exhaustion is going on, the air included in the vessel, and whose pressure upon the water is no longer balanced by that of the exterior air, dilates itself, and gives birth to a jet which rises under the receiver. (See fig. 5.) But the most interesting experiment relative to this object is that of Boyle, and of Mariotte, to show that the air contracts itself nearly in the ratio of the weights with which it is pressed. These kinds of experiments merit the preference, since they are not confined to merely proving the existence of a phenomenon, but make known also how it exists, by determining the law to which it is subject. Take a glass tube a b (Plate Pneumatics, fig. 1.) bent into two branches, the shortest of which is about twelve inches high; it must be equally thick throughout, and hermetically sealed at its extremity b. The other branch, which is open at a, should be at least five feet, but if it were eight feet in height, so much the better. The whole is fixed upon a plate which carries divisions adapted to the two tubes. First, let there be poured into the bent part a little mercury, to obtain a line of level, xz, that we may estimate the number of degrees comprised between that line and the superior extremity of the shortest branch. In this state of things the air which occupies that branch maintains an equilibrium, by its elasticity, with the pressure of the column of atmospheric air gravitating in the other branch, and whose pressure is transmitted by means of the mercury comprised in the inferior curvature. This pressure, as we have seen in the article BAROMETER, is equal to that of a mercurial column of about twenty-nine or thirty inches in height. Afterwards, let mercury be poured into the longest branch, and at the same time the air in the other branch will be condensed; by the excess of the resulting pressure, the mercury will rise in the shorter branch until an equilibrium is again produced. Then measure, on one part, the length of that column of compressed air, and on the other the excess of the column of mercury contained in the longest branch, above that which occupies the shortest. We will suppose, for more simplicity, that this excess is equal to thirty inches; in that case, we shall find that the column of compressed air is reduced to the half of the height which is occupi